Mobile computing, which includes access to the Internet on the move, is getting more popular nowadays. Hand phones are increasing in functionality and usage of wireless LANs and the Internet is growing. Nowadays, more and more portable terminals have the ability to connect to the Internet using a wide range of access technologies, such as Third Generation (3G) cellular networks, General Packet Radio Service (GPRS), IEEE 802.11a/b/g and Bluetooth. Mobility refers to the fact that the network connection is available even when a node moves.
Various portable computing devices such as handheld mobiles, laptops and personal digital assistants (PDAs) with enhanced networking capabilities have increased the demand for seamless communication both in wired and wireless networks. Increased use of multimedia contents such as video conferencing makes seamless communication become an essential and required feature in mobile connections. Practical mobility management should provide seamless handoff where the user does not observe communication disruptions. Today's mobile data networks commonly consist of several wireless overlapping networks, supporting different data rates and geographical coverage and can only be accessed via media specific air interfaces.
Each time a mobile node changes a cell, a subnet or a network, coverage responsibilities have to be switched between respective serving access nodes. To provide seamless roaming among these heterogeneous networks, mobile nodes have to be capable of connecting to various access networks. Moreover, a comprehensive mutually agreed global mobile network architecture is required to manage the macro mobility. At the same time, with the wide range of wireless access technologies available, portable information terminals (multimode nodes) with multiple access interfaces are starting to appear. These include dual-mode hand phones, laptops and Personal Digital Assistants.
The usefulness of attaching to the Internet using different access technologies at different time or even simultaneously, has been described in various documents. However, protocols embedded in multi-mode nodes available in today's consumers market do not provide for seamless switching between heterogeneous interfaces. There are also some issues to be resolved when seamless switching between heterogeneous interfaces. With the multiplicity of overlapping available wireless networks, the availibility of a network in question may be transient or stable from the view of the mobile node.
For example, when a mobile node is roaming along the fringes of the wireless networks, the transient nature of the network coverage may cause spurious and needless handover between networks and media interfaces even though the stability for the practical use is not ensured. The overheads incurred in such handover would disrupt the user experience and cost unnecessary loading on the networks.
One solution of the following Patent Document 1 proposes a scheme to support service resumption when the mobile terminal roams among the coverage areas of wireless networks by tagging services with a local or global. Global services may be resumed when roaming by reinitiating contact with the same server via a different gateway.
Another solution of the following Patent Document 2 utilizes a Mobile Switching Centre in the network to send duplicated frames to a roaming mobile terminal with synchronized timing in order to affect soft handover. Duplicated frames sent from the terminal is also processed and removed by the Mobile Switching Centre.
Furthermore, another solution of the following Patent Document 3 proposes that a cellular timing receiver is installed in the wireless LAN access point in order for the cellular network and the wireless LAN to be synchronized so that soft handover may be performed.
[Patent Document 1] U.S. Patent Publication 20030112789A1
[Patent Document 2] U.S. Patent Publication 20040213279A1
[Patent Document 3] U.S. Patent Publication 20030030101A1
The above-mentioned solution disclosed in the Patent Document 1, however, does not optimize the handover process and also suffers from the fringe roaming problem described previously. The above-mentioned solution disclosed in the Patent Document 2, however, would not work across heterogeneous networks especially if the networks belong to different administrative domains or Service Providers. The above-mentioned solution disclosed in the Patent Document 3, however, would require participating cellular and wireless LAN networks to be synchronized, reducing the total user capacity. Furthermore, there may also be problems implementing this solution across different administrative domains, as in case of the above-mentioned solution disclosed in the Patent Document 2.
The above prior arts can be summarized as follows.                A mobile terminal cannot maintain the seamless connectivity, especially when performing handover across heterogeneous networks.        It is difficult that a mobile terminal determines whether an access network is transient or stable so as to perform efficient handover for the entire network.        There appears the fringe roaming problem causing unnecessary handover, especially when a mobile node is roaming along the fringe of the network coverage.        